1. Field of the Invention
The present invention relates to an optical film, a polarizing plate using the optical film, and an image display apparatus using the optical film or the polarizing plate.
2. Description of the Related Art
Optical films, notably an antireflective film, are generally placed at the outermost surfaces of displays, such as a cathode-ray tube (CRT) display, a plasma display panel (PDP), an electroluminescent display (ELD) and a liquid crystal display (LCD), for the purpose of preventing a contrast drop from occurring by reflections of outside light from the display surface and ambient-light reflection in image display apparatus. Therefore, the antireflective film is required to have not only high antireflection capabilities but also high transmittance, high physical strength (e.g. high resistance to scratching), chemical resistance and weather resistance (including resistances to moisture, heat and light).
The antireflective film prevents reflections from occurring by utilizing optical interference of a low refractive index layer that is thin film measuring 200 nm or below in thickness and provided at least at the outermost surface. However, in the case of a single-layer thin-film interference type which inhibits reflection by a low-refractive index layer of the simplest structure formed of one layer, any low-refractive-index materials suitable for practical use, which can satisfy a 5° specular reflection factor of 0.5% or below and have neutral color, high scratch resistance, chemical resistance and weather resistance, are not found yet.
On the other hand, antireflective films of multilayer thin-film interference type which can prevent reflections through multilayer optical interference are known as those for ensuring a 5° specular reflection factor of 0.5% or below, and examples of an antireflective film of such a type include an antireflective film of double-layer thin-film interference type wherein a high refractive index layer is formed between a transparent support and a low refractive index layer and an antireflective film of triple-layer thin-film interference type wherein a medium refractive index layer and a high refractive index layer are formed in the order of mention between a transparent support and a low refractive index layer. And the antireflective film of triple-layer thin-film interference type is especially favorable in the case where the prevention of reflection over a wide range of wavelengths and the attainment of a low reflectance while reducing reflection colors are required.
As thin-film layers used in an antireflective film of multilayer interference type (e.g. a high refractive index layer, a medium refractive index layer, a low refractive index layer), a multilayer film formed by lamination of transparent thin-film layers of metal oxides has been widely adopted so far. The transparent thin films of metal oxides are usually formed by a chemical vapor deposition (CVD) method, a physical vapor deposition (PVD) method, a vacuum evaporation method or a sputtering method. However, the methods of forming transparent thin films of metal oxides by vapor deposition or sputtering are low in productivity and unsuitable for mass production. Therefore, there are propositions to adopt wet film-forming methods high in productivity, notably a method of forming film by a coating process.
On the other hand, in order to reduce a viewability drop caused by reflections of extraneous light from a display surface and the reflection of a fluorescent lamp or the like in a display, an antiglare layer having an uneven surface shape is used. The antiglare layer having an uneven surface shape is formed e.g. by coating the surface of a transparent support with a resin containing fine particles of an inorganic oxide like silica or organic resin beads. When a display is viewed in a lighted room, the ambient-light reflection in the display can be reduced by the use of an antiglare layer, but there occurs a phenomenon that the screen to be essentially black in black-display mode looks whitish because of surface scattering of extraneous light and suffers from a drop in contrast (hereafter referred to as deterioration in denseness of black). Therefore, simultaneous achievement of both reduction in reflection and retention of high denseness of black is aimed at by providing one antireflective layer on the antiglare layer.
For further improvements in the reflection and the denseness of black, it is favorable to form an antireflective layer of double-layer or triple-layer thin-film interference type on an antiglare layer. However, when it is tried to coat an uneven surface of an antiglare layer with an antireflective layer on a nanometer scale, there may be cases where the uneven surface of the antiglare layer results in uneven coating and cissing of the antireflective layer, and thereby the thickness of the antireflective layer comes to fluctuate with positions on the antiglare layer and occurrence of optimum optical interference becomes impossible; as a consequence, a sufficient drop in reflectance does not occur. Accordingly, it is difficult to achieve both low reflection and high antiglare properties at the same time.
For the purpose of improving uneven coating and cissing produced on the uneven surface of an antiglare layer, disclosures of the aging treatment subsequent to the surface treatment of an antiglare layer (JP-A-2006-145737) and the provision of an interlayer between an antiglare layer and an antireflective layer (JP-A-2006-145587) are made. In addition, it is tried to make fine particles of hollow spherical silica in a low refractive index layer be present in greater amounts in concave portions than in convex portions of the uneven surface of an antiglare layer (JP-A-2006-146027).
However, these optical films heretofore proposed are insufficient to lower the reflectance. Moreover, since antiglare films currently in use have microscopic asperities on their surfaces, they have a problem that light scattering from their surfaces worsens denseness of black as compared with clear films having even surfaces.